The EPA on Friday said Volkswagen and Audi had allegedly skirted federal emissions requirements in roughly 482,000 diesel vehicles in the U.S. since 2009 by using software that circumvents emissions testing for certain air pollutants.

The VW cars under investigation emit up to 40 times the national standard for nitrogen oxide, which is linked to asthma and lung illnesses, the EPA said.

"Using a defeat device in cars to evade clean air standards is illegal and a threat to public health," said Cynthia Giles, assistant administrator for the Office of Enforcement and Compliance Assurance, in a statement.

The EPA said it wants the cars brought into compliance.

On Sunday, Volkswagen CEO Martin Winterkorn apologized to consumers and said the company has ordered an external investigation to "reverse the damage this has caused."

"I personally am deeply sorry that we have broken the trust of our customers and the public," Winterkorn said in a statement. "We will cooperate fully with the responsible agencies, with transparency and urgency, to clearly, openly and completely establish all the facts of this case.

"Volkswagen has ordered an external investigation of this matter. We do not and will not tolerate violations of any kind of our internal rules or of the law."

Volkswagen of America on Sunday told Edmunds that consumers "should be aware that this is not a safety-related issue. Owners of these vehicles do not need to take any action at this time."

Audi of America deferred all questions about the EPA announcement to parent Volkswagen.

"This is a notice of non-compliance that needs to be addressed," VW said. "Volkswagen will develop a remedy in coordination with EPA and CARB."

The affected vehicles have not been recalled yet and a remedy for the noncompliance issue has not been announced.

Volkswagen is not allowed to sell 2016 vehicles with the 2.0-liter diesel engine at this point.

The EPA said VW may be liable for civil penalties for the alleged violations.

VW has been touting its "clean diesel" vehicles that get high fuel economy. American car shoppers are becoming more open to diesel vehicles as the price of diesel fuel more closely aligns with that of regular gasoline.

The AAA Daily Fuel Gauge Report on Monday pegged the average price for a gallon of diesel fuel at $2.50 versus $2.28 for regular gasoline.

Edmunds says: Car shoppers looking for a diesel vehicle have other options besides offerings from Volkswagen and Audi. Owners of the affected VW and Audi diesel vehicles will be notified when these vehicles are recalled.